This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/FI99/00553 which has an International filing date of Jun. 22, 1999, which designated the United States of America.
1. Field of the Invention
The invention relates to a micromechanical AC voltage reference system.
The invention also concerns a micromechanical DC voltage reference system.
The invention further concerns an AC/DC converter based on an identical mechanical structure.
Possibilities of linking electrical quantities to mechanical quantities by means of a linear capacitive voltage transducer are contemplated, e.g., in a reference publication xe2x80x9cF. Cabiati, xe2x80x9cLinking Electrical With Mechanical Quantities Through Electro-Mechanical Resonance,xe2x80x9d in Conference Digest of CPEM96, pp. 610-611, 1996xe2x80x9d. These investigations have been performed into large electromechanical structures subject to mechanical instability factors and requiring a high bias voltage for operation.
It is a primary application of an AC/DC converter to measure the RMS value of an AC signal using a known DC signal as the reference. The most accurate state-of-the-art AC/DC converters are electrothermic transfer standards based on the comparison of ohmic heating caused by the applied AC and DC signals, respectively. One disadvantage of electrothermic converters is that they impose a heavy load on the signal source connected to their input.
A problem hampering fully electronic AC/DC converters is their relatively low maximum operating frequency and the resulting degradation of accuracy.
AC/DC converters may also be realized using micromechanical electrode structures operating based on an electrostatic force. The use of a silicon micromechanical capacitive electrode structure in the RMS amplitude measurement of an AC voltage signal is discussed in reference publication by xe2x80x9cB. P. van Driexc3xanhuizen and R. F. Wolffenbuttel, entitled xe2x80x9cIntegrated Micromachined Electrostatic True RMS-to-DC Converter,xe2x80x9d in IEEE Transactions on Instrumentation and Measurement, Vol. 44, No. 2, pp. 370-373, 1995xe2x80x9d. The publication describes a bridge structure made from polycrystalline silicon by surface micromechanical techniques. The structure includes an electrode pair in which one electrode is elastically suspended. An AC voltage applied between the electrodes causes a change in the interelectrode capacitance, the magnitude of which is measured to determine the RMS value of the applied AC voltage. According to this publication, the conversion of the measured capacitance into the respective AC RMS voltage was made using an electronic circuit. Hence, the accuracy of the AC RMS voltage measurement is dependent not only on the qualities of the micromechanical structure, but also on those of the above-mentioned electronic circuit used.
It is an object of the present invention to overcome the disadvantages of the above-described prior art techniques and to provide an entirely novel type of micromechanical AC and DC voltage reference system.
Furthermore, the invention disclosed herein is free from the shortcomings of the above-described micromechanically constructed AC/DC converter based on the measurement of an electrostatic force.
The goal of the invention is achieved by virtue of a system based on a micromechanical structure comprising at least one electrode pair that further comprises a first electrode and a second electrode disposed at a distance from each other so as to make at least one of said electrodes movable against a spring force, said system further including means for applying an AC signal over said electrodes for establishing an electrostatic force advantageously at a frequency which is substantially higher than the effective mechanical resonant frequency of the movable electrode and said system further including means for detecting the AC voltage acting over the electrodes thus creating the AC voltage reference. In an AC/DC converter based on a preferred embodiment of the bridge structure according to the invention, the electrostatic forces created by the AC signal and the DC signal are compared directly with each other using an embodiment of the micromechanical electrode structure. The electrical circuit measuring the capacitive voltage division ratio senses the force balance.
More specifically, the AC voltage standard according to the present invention is a system that comprises at least one micromechanically fabricated electrode pair. The pair includes a first electrode and a second electrode adapted to face each other so that the electrodes are disposed at a distance from each other, so that at least one of the electrodes is movable against a spring force. Means are provided for applying an AC signal over the electrodes for establishing an electrostatic force advantageously at a frequency substantially higher than the effective mechanical resonant frequency of the movable electrode. The system also includes an apparatus for detecting the AC voltage applied between said electrodes, thus forming an AC voltage reference.
Furthermore, the DC voltage standard according to the present invention comprises at least one micromechanically fabricated electrode pair. The electrode pair includes first and second electrodes facing each other so that the electrodes are disposed at a distance from each other, and at least one of the electrodes is movable against a spring force. Means are provided for applying an electrical charge between the electrodes in order to deviate the electrodes from their mutual equilibrium position, together with means for detecting the DC voltage applied between the electrodes, thus forming an DC voltage reference.
Still further, the AC/DC voltage transfer standard system, that is, an AC/DC converter, according to the present invention, comprises at least two micromechanically fabricated electrode pairs disposed at a distance from each other so that at least one of the electrodes is movable against a spring force. Means are provided for feeding a DC signal on one electrode of the first electrode pair. Also included are means for feeding an AC signal on one electrode of the second electrode pair, as well as means for detecting the position of the movable electrode or, alternatively, the force required to maintain the position of the electrode, to compare the RMS values of the applied AC and DC voltages.
The invention offers significant benefits.
Voltage standards according to the invention based on the advantageous mechanical and geometrical properties of monocrystalline silicon can exhibit an extremely high stability. Also the production costs of such voltage standards can be brought to a very reasonable level.
As the electronic circuit of an AC/DC converter according to a preferred embodiment of the invention essentially acts as a zero-position indicator, the problems occurring from, e.g., nonlinearities of electronics are avoided and thus an entirely novel type of AD/DC converter is accomplished.